As an example of antenna device according to the related art, there is a microstrip antenna (patch antenna) in which a radiating conductor element and a ground conductor plate are provided facing each other across a dielectric that is thin relative to wavelength, and a parasitic conductor element is provided on the radiating surface side of the radiating conductor element. See, for example, Japanese Unexamined Patent Application Publication No. 55-93305.
The antenna device according to Japanese Unexamined Patent Application Publication No. 55-93305 achieves bandwidth enhancement by exploiting magnetic coupling between the radiating conductor element and the parasitic conductor element.